Building a wooden sea kayak nowadays is a quite complex procedure.
To simplify the procedure a few companies/people offering free or paid building plans. That gives you at least a good starting point instead of designing everything from scratch.
To make the long story short I & my colleague Vasilis choose to build the CNC Vember Expedition sea kayak. It is a British style kayak.
The boat comes into two versions; the normal and the expedition. We did the second one which extends the length by 10% to 5.35m (17.53 feet) while maintaining the same beam and stability characteristics. The construction follows the Vember Build Manual, but with the transverse temporary forms spaced at 330mm rather than 300mm.